The invention relates to a process for the production of dinitrotoluene by nitration of toluene with nitrating acid (mixture of nitric acid and sulfuric acid) in which, in a first stage, the toluene is converted to mononitrotoluene (MNT) and then the mononitrotoluene is converted in a second stage to dinitrotoluene (DNT).
Dinitrotoluene (DNT) is an intermediate for the production of toluene diisocyanate (TDI), which is an important starting material produced on an industrial scale for the production of polyurethanes.
The production of dinitrotoluene by nitration of toluene with nitrating acid (mixture of nitric acid and sulfuric acid) is known and has been described many times (See, e.g., Ullmann's Enzyklopedie der technischen Chemie, 4th edition, volume 17, pages 391 ff, 1979, Verlag Chemie Weinheim/New York). The industrial production takes place, as described, e.g., in H. Hermann, J. Gebauer, P. Konieczny, “Industrial Nitration of Toluene to Dinitrotoluene” in ACS-Symposium, Series 623, 234-249, 1996, ed. L. F. Albright, R. V. C. Carr, R. J. Schmitt, predominantly isothermally with nitric acid in the presence of sulfuric acid as catalyst continuously in two stages, such that                a) the reaction mixture obtained in the dinitration (nitration of MNT to DNT) is separated by phase separation and the used acid is fortified again with nitric acid and then mixed with toluene and fed into the mononitration (nitration of toluene to MNT), and        b) the reaction mixture from the mononitration is separated in a separating stage on completion of the reaction into an organic phase containing the mononitrotoluene and an aqueous phase predominantly containing the sulfuric acid (“spent acid”), and        c) the organic phase containing the mononitrotoluene obtained in b) is fed into the dinitration and the mononitrotoluene is reacted there with nitric acid in the presence of sulfuric acid to form dinitrotoluene.        
To achieve commercial specifications, the crude DNT thus obtained is generally treated in downstream stages, predominantly washes, and thus largely freed of dissolved sulfuric and nitric acid contents as well as of secondary components formed in the reaction stages, e.g., cresols and their degradation products. Typical commercial DNT products have DNT contents of >98.5 wt. %, less than 0.1 wt. % mononitrotoluene, less than 0.1 wt. % trinitrotoluene and less than 0.1 wt. % of other secondary components, based on the weight of the DNT product mixture, with DNT yields of >98% and toluene conversions of >99.9%. The weight ratio of the 2,4- and 2,6-DNT isomers in total to the 2,3-, 3,4-, 2,5- and 3,5-DNT isomers in total is also essential. According to commercial specifications, the content of the 2,4- and 2,6-DNT isomers in total in the crude DNT is >95 wt. %, based on the weight of the crude DNT. The content of 2,4-DNT is preferably 79.0-81.0 wt. %, based on the sum of the weights of 2,4-DNT and 2,6-DNT. Accordingly, the content of 2,6-DNT is 19.0-21.0 wt. %, based on the sum of the weights of 2,4-DNT and 2,6-DNT.
In addition to the crude DNT, spent acid is obtained in the separation of the reaction mixture obtained in the mononitration, which leaves the system as a second mass flow. The spent acid generally has a sulfuric acid content of 70-74 wt. % and generally contains >0.1, preferably >0.2 to 1.5 wt. % of unreacted nitric acid, nitrosulfonic acid from oxidation processes occurring in secondary reactions, >0.2 wt. % of MNT, which was not separated off in the phase separation, and generally water in a concentration range of >26 to <30 wt. % (comprising the water introduced with the fresh sulfuric acid fed into the process, water contained in the nitric acid and water formed during the nitrations of the toluene and mononitrotoluene), based in each case on the weight of the spent acid.
In the two-stage isothermal nitration of toluene to dinitrotoluene with nitric acid in the presence of sulfuric acid, both the use of highly concentrated nitric acid (DE 10 2004 005 913 A) and the use of azeotropic and sub-azeotropic nitric acid (EP 0 903 336 A2) in a concentration range of >57 to 69 wt. % are described. Concentrations of >69 wt. % require increased, cost-intensive technical input, which is substantially caused by overcoming the azeotrope.
The concentration of the sulfuric acid used, which acts as a catalyst and as a water-removing agent, is influenced in industrial DNT plants substantially by the concentration of the nitric acid fed into the process and the reaction conditions in the mononitration stage.
It is known that, under suitable conditions, nitric acid is capable not only of nitrating but also of oxidizing the organic compounds in the isothermal two-stage nitration of toluene to dinitrotoluene with nitric acid in the presence of sulfuric acid. Possible compounds obtained by the oxidation of toluene, MNT or DNT are cresols, phenols and their nitration products. A low concentration of sulfuric acid reinforces the oxidizing tendency of nitric acid, so that the content of organic by-products in the reaction mixture increases as the sulfuric acid concentration decreases. On the other hand, as the sulfuric acid concentration falls, the rate of the nitrating reactions desired in the mononitration decreases. Both phenomena lead in industrial DNT plants to a lower limit for the sulfuric acid concentration at which this process can be operated economically. This limit is generally about 70 wt. % (EP 0 903 336 A2).
This limit value has consequences for the sulfuric acid concentration of the spent acid obtained in the phase separation of the dinitration stage and recycled to the mononitration stage. The concentration of H2SO4 in the spent acid from the dinitration stage is generally greater than 80 wt. %, on the one hand to limit the sulfuric acid throughputs through the mononitration and on the other hand to guarantee a high reaction rate for the nitration in the dinitration stage. Because the concentration of sulfuric acid also has a strong influence on the rate of the desired reactions in the dinitration stage, falling concentrations lead to lower reaction rates.
To meet the concentration requirements outlined and the specifications of an industrial DNT mentioned above, sulfuric acids with concentrations of from 93 to 98 wt. % H2SO4 or higher, based on the weight of the sulfuric acid, are generally used in industrial DNT plants using the isothermal two-stage process. When sub-azeotropic nitric acids in the range of 60-65 wt. % HNO3, based on the weight of the nitric acid, are used, sulfuric acids with concentrations of more than 95 wt. % H2SO4, based on the weight of the sulfuric acid, are generally employed. Deviating from this, EP 0 903 336 A2 teaches the use of 86-91 wt. %, preferably 87-89 wt. %, sulfuric acid. The acids used can either be freshly produced or obtained by concentrating the spent acid from the phase separation of the reaction mixture from the mononitration in a concentrating plant.
In addition to this standard process for the 2-stage continuous isothermal nitration of toluene with nitric acid in the presence of sulfuric acid, it has also been proposed to carry out the nitration of toluene to dinitrotoluene with nitrating acid continuously in three stages (EP 903 336 A) or adiabatically in one or two stages in such a way that, as described in EP 597 361 A and EP 696 570 A, all of the heat of reaction from the nitration of the toluene to DNT or only from the DNT stage, as described in EP 696 571, is used to separate off the water of reaction from the nitration and the water introduced into the spent acid by the nitric acid. In addition, it was proposed in U.S. Pat. Nos. 5,948,944 A and 2,362,743 A to carry out the nitration of toluene to DNT only in nitric acid as the reaction medium, thus avoiding the use of sulfuric acid.
In all processes for the production of DNT by nitration of toluene with nitric acid, it is a prerequisite for an economic operating of the process that the spent acids forming in the processes can be reprocessed in such a way that they can be fed into the reaction process again as a reaction medium, as described, e.g., in EP 155 586 A and U.S. Pat. No. 5,275,701 A.
Essential considerations for the selection of a nitrating process, however, are also its inherent process safety, the robustness with which it can be operated, the selectivity and completeness with which the toluene can be converted to dinitrotoluene, and the specific use of nitric acid, which is necessary for the conversion of the toluene to dinitrotoluene.
In view of the above criteria, the industrial-scale production of dinitrotoluene from toluene with nitric acid predominantly takes place according to the so-called nitrating acid or mixed acid process, in which the toluene is reacted continuously with nitric acid in the presence of sulfuric acid in two isothermally operated reaction stages to form dinitrotoluene. In this nitrating acid process,                a) the reaction mixture obtained in the dinitration (nitration of MNT to DNT) is separated by phase separation and the used acid thus obtained is fortified again with nitric acid and then mixed with toluene and fed into the mononitration (nitration of toluene to MNT), and        b) the reaction mixture from the mononitration is separated in a separating stage on completion of the reaction into an organic phase containing the mononitrotoluene and an aqueous phase predominantly containing the sulfuric acid (“spent acid”), and        c) the organic phase obtained in b) containing the mononitrotoluene is fed into the dinitration and the mononitrotoluene is reacted there with nitric acid in the presence of sulfuric acid to form dinitrotoluene.        
The selectivity of the toluene reaction is substantially influenced by the sulfuric acid concentration in the two reaction stages of the process. As set out above, sulfuric acid concentrations of <70 wt. % H2SO4, based on the weight of the sulfuric acid, in the mononitration lead to an increased oxidizing tendency of the nitric acid, and as a result of the oxidation of toluene, MNT or DNT, cresols, phenols and their nitro and degradation products are obtained. On the other hand, sulfuric acid concentrations that are too high in the dinitration stage lead to an increased formation of trinitrotoluene because of the nitric acid that is constantly present.
The desired completeness of the reaction in the reaction stages is also influenced, with a given residence time, by the sulfuric acid concentration and the reaction temperature selected. It is additionally dependent on the nitric acid concentration in the sulfuric acid phase. It also depends on the interfaces produced in the reaction stages, since the reaction mixture in both nitration stages tends to decompose into an organic phase containing only traces of acids and a sulfuric acid phase.
A high specific use of nitric acid, based on the components to be nitrated, does promote their reaction but, on the other hand, it leads to marked nitric acid loadings of the spent acid that is obtained in the phase separation of the mononitration stage or the DNT which is removed in the phase separation of the dinitration stage. It also leads to significant quantities of unreacted nitric acid which have to be removed from both mass flows and reprocessed in subsequent stages and can then be fed back into the reaction stages.
There has been no lack of attempts to improve the isothermal two-stage reaction of toluene with nitric acid in the presence of sulfuric acid with respect to its economic efficiency because the production of dinitrotoluene takes place industrially on such a large scale that even small economic improvements in this important industrial process are of great economic interest.
EP 903 336 A teaches the use of preferably 87-89 wt. % sulfuric acid, which is produced by reprocessing the spent acid from the mononitration and can be obtained with significantly lower expenditure as sulfuric acids with contents of >89 wt. %. The lower sulfuric acid concentration is taken into account by the fact that the process is carried out in three nitration stages rather than two. The organic phase containing the dinitrotoluene obtained in the phase separation of the second stage is fed into a third reaction stage, known as the polishing zone, and is reacted in the polishing zone with a mixed acid containing an aliquot proportion of nitric acid and all of the fresh sulfuric acid fed into the process. The basis of this process is that the sulfuric acid fed into the polishing zone is diluted only by the water content of the aliquot nitric acid feed and the water of reaction formed by the residual nitration taking place in the polishing zone. Acid strengths that are even higher than the acid strengths in a standard process are thus achieved in the polishing zone. A disadvantage of this three stage process, besides the additional investments and operating costs of the third stage, is that the polishing zone has only one reactor. For the complete conversion of the MNT, therefore, so much nitric acid has to be added that the spent acid from the third stage subsequently obtained has a nitric acid content of about 0.4 wt. %. Such nitric acid content, in conjunction with the increased sulfuric acid concentration, presents the risk of increased TNT formation (U.S. Pat. No. 3,157,706 A) on the one hand and, according to experience, lead to nitric acid contents in the discharged DNT of significantly >0.4 wt. %, and thus to significant quantities of unreacted nitric acid, on the other hand.
Minimizing nitric acid losses through the DNT discharge is the goal sought in EP 279 312 A2. EP 279 312 A2 teaches a process for the separation of sulfuric acid and nitric acid from the dinitrotoluenes containing sulfuric acid and nitric acid obtained in the dinitration of toluene with mixed acid. In this disclosed process, the dinitrotoluenes obtained after separating off the greater part of sulfuric acid and nitric acid, which still contain up to 6 wt. % sulfuric acid and up to 5 wt. % nitric acid, are mixed with up to 10 wt. % water, based on the quantity of dinitrotoluenes, and the aqueous phase containing sulfuric and nitric acid that then separates out is removed. The DNT wash is performed in one or more stages in this process.
Improved acid recovery compared with EP 279 312 A2 is taught by EP 736 514 A1. EP 736 514 A1 discloses a process for the removal and recovery of nitric acid, sulfuric acid and nitrogen oxides from the crude dinitrotoluenes formed in the nitration of toluene or mononitrotoluenes after removal of the nitrating acid. In this disclosed process, the crude dinitrotoluenes are extracted in counter-current with a dilute aqueous solution of nitric acid, sulfuric acid and nitrous acid in multiple stages, particularly two to four stages, the volume ratio of the dinitrotoluenes to the aqueous solution being 1:3 to 10:1, preferably 1:1 to 4:1 in each case. The aqueous extract is recycled into the nitration directly or after being concentrated.
In all the extraction stages it is useful to work at a temperature above the melting point of the dinitrotoluenes. The density of the aqueous solution should be different from, preferably lower than, that of the dinitrotoluenes in all the stages. The dilute aqueous solution is usefully circulated within each extraction stage. The desired volume ratio of the dinitrotoluenes to the dilute aqueous solution of nitric acid, sulfuric acid and nitrous acid can be adjusted by adding fresh water into the extraction circulation of the dilute solution from the last extraction stage. In particular, the condensate that forms when the aqueous extract is being concentrated is added. The aqueous solution drawn off from the first extraction stage is a nitric acid/sulfuric acid mixture with 25 to 40 wt. % total acid. This is concentrated alone, or preferably with the nitric acid from treatment of the final mononitrotoluene acid, to a total acid content of 65 wt. %, calculated as HNO3.
With a view to minimizing unused nitric acid, the nitric acid content of the spent acid obtained in the phase separation of the mononitration stage is also the subject of numerous investigations.
U.S. Pat. No. 2,947,791 A teaches an improved continuous process for the nitration of toluene in which equimolar quantities of nitric acid (a component of the nitrating acid of nitric and sulfuric acid, containing 50-60 wt. % sulfuric acid, 20-40 wt. % nitric acid and 10-20 wt. % water) and toluene are reacted at 50 to 100° C. in a well agitated system made up of two reactors connected in series in such a way that 0.4 to 0.7 moles of toluene per mole of nitric acid are fed into the first reactor and the remaining quantity of toluene is then added to the reaction mixture leaving the first reactor and reacted in the second reactor.
As a result of the toluene split performed, a toluene conversion of >95% is obtained in the mononitration stage according to the teaching of U.S. Pat. No. 2,947,791 A, with low contents of nitrogen oxides and nitric acid in the aqueous phase. In the following dinitration, a 5-10% molar excess of nitric acid is then always used.
U.S. Pat. No. 2,475,095 A teaches that residual quantities of toluene in the mononitration reaction mixture make the subsequent phase separation of the reaction mixture more difficult. U.S. Pat. No. 2,475,095 A therefore teaches that an excess of nitric acid compared with toluene is already fed into the mononitration. In this case, the spent acid from the mononitration stage is said to have a nitric acid content of 1 wt. %.
U.S. Pat. No. 4,496,782 A is also based on a nitric acid excess in the mononitration and thus a significant content of nitric acid in the spent acid from the mononitration stage. To use these contents, U.S. Pat. No. 4,496,782 A teaches the addition of additional nitric acid to the spent acid from the mononitration to nitric acid contents of >2 wt. %, and then reacting this nitric acid with an aliquot amount of mononitrotoluene in a stirred reactor adiabatically at temperatures of >110° C. in such a way that, in the subsequent phase separation, spent acids with nitric acid contents of <0.25 wt. % are obtained. In addition to increased technical input, this process presents safety risks.
DE 10 2004 005 913 A1 emphasizes the technical input required for the two-stage isothermal reaction of toluene with nitric acid in the presence of sulfuric acid. According to the teachings of DE 10 2004 005 913 A1, it is essential to reduce this technical input compared with the prior art (which, according to DE 10 2004 005 913 A1, is two- to four-stage agitated vessel cascades in each of the mononitration and dinitration stages). DE 10 2004 005 913 A teaches the use of one agitated vessel in the mononitration stage and the use of two agitated vessels connected in cascade in the dinitration stage.
DE 10 2004 005 913 A1 also teaches an incomplete toluene conversion with, at the same time, significant nitric acid contents in the spent acid (0.96 wt. % in the example illustrating the process taught therein) in the mononitration stage and the complete conversion in the dinitration stage, which is achieved by an appropriate excess of nitric acid.
DE 10 2004 005 913 A1 counters the problem of the significant quantities of unreacted nitric acid withdrawn from the system in the spent acid from the mononitration or dinitration with the following note: “To minimize losses of nitric acid, which does not become nitric acid converted into the end product, as described e.g. in EP 0 736 514, the nitric acid from the wash of the crude DNT is recycled into the nitration as weak acid with a total acid content of 23.73 to 40% total acid together with the nitric acid from the waste gas wash and from the stripping of the spent acid, directly or after being concentrated”. DE 10 2004 005 913 therefore replaces the lower expenditure in the reaction stages with increased expenditure in the DNT and spent acid workup stages, and also contains the risk of further reactions in the mononitration stage running out of control and an increased trinitrotoluene (TNT) formation in the dinitration stage.